1. Field of the Invention
The present invention relates to an image processing device, an image forming system, an image processing method, and a computer program product.
2. Description of the Related Art
Conventionally, there are image forming apparatuses that are provided with clear toner that is colorless toner that contains no color materials in addition to the four color toners of cyan (C), magenta (M), yellow (Y), and black (K). The toner image, formed by using the above-described clear toner, is fixed to the recording sheet on which images have been formed by using the CMYK toner and, as a result, the visual effect or the tactual effect (referred to as the surface effect) is applied to the surface of the recording sheet. The applied surface effect differs depending on what kind of toner image is formed and how it is fixed by using the clear toner. Some of the surface effects simply give glossiness to the recording surface, and other surface effects prevent glossiness. Furthermore, there is a need for the surface effect to, not only give the surface effect to the entire surface of a recording sheet but also give the surface effect only to part of it or apply textures or watermarks by using clear toner. Furthermore, there is sometimes demand for the protection of the surface of a recording sheet. Furthermore, there are surface effects that may be applied by controlling the fixing performance to the recording surface or by performing post-processing with a dedicated post-processing device, such as glosser or low-temperature fixing device.
Furthermore, there are image forming apparatuses that are provided with white toner that contains, as the principal component, white pigment and a binder resin component and that does not contain any color material components other than the single white color in addition to the toners that correspond to the process colors that are the four CMYK colors. In recent years, the white toner has been used as a color material that adds various values, such as printing to transparent recording medium, fabric, or colored paper, in addition to faithful color reproduction using the process colors.
There are known devices that use the fifth toner as the color material that adds the above-described various values other than the CMYK process colors, and there is a known screen technology for performing the tone number conversion by using the large number of lines when images are formed by using multivalued tone image data where the clear toner is used as the fifth plate. For example, Japanese Patent No. 4701988 discloses an image processing device that uses dither with the large number of lines for the clear toner rather than the process colors for the purpose of printing a desirable color image with a smooth image surface even if the transparent toner image is misaligned from the color toner image.
For example, according to Japanese Patent No. 4701988, the visual image quality of transparent toner images is not much affected by a certain degree of deviation or unnaturalness that might be present in the output image. According to the descriptions, it is preferable to use a dot-centered type dither method for color toner images and use an error diffusion method or a dither method with the large number of lines for transparent toner images.
Furthermore, Japanese Patent No. 5217548 discloses the technology for preventing uneven glossiness that occurs due to the difference between the glossiness of a transparent toner area and the glossiness of a chromatic color toner area or moire patterns due to the occurrence of periodic toner scattering at the position where the transparent toner and the chromatic color toner are superimposed. According to Japanese Patent No. 5217548, the dither with the large number of lines is used for the clear toner rather than the process colors, transfer is conducted such that the clear toner plate is located on the top and, in the printing mode for reducing uneven glossiness, the percentage of the image area with the maximum density is decreased.
However, the above-described conventional technologies have the following problem. With regard to the image forming devices that are provided with special toner (clear toner, white toner, or special color toner) other than the process colors, consideration is not given to the optimization of a special-toner screen that is superimposed on multiple process-color toner screens. Therefore, in some halftone modes, unexpected moire or uneven glossiness occurs depending on the superimposition condition of the process-color toner screen and the special-toner screen. Specifically, there is a problem in that image outputs may not be obtained in accordance with the intended purpose of the halftone mode.
Furthermore, Japanese Patent No. 4701988 and Japanese Patent No. 5217548 do not consider the optimization of a special-toner screen that is superimposed on multiple process-color toner screens. Therefore, in some halftone modes, unexpected moire or uneven glossiness occurs depending on the superimposition condition of the process-color toner screen and the special-toner screen, and image outputs may not be obtained in accordance with the intended purpose of the halftone mode.
In view of the foregoing, there is a need to obtain image outputs in accordance with the intended purpose of the halftone mode when the special toner plate is superimposed on the process-color toner plate.